In the electrolytic cleaning and recovery of metals, the surface quality of the cathode plate is important. By affecting the adhesion properties of the surface, a cathode plate of a better quality is obtained. In the electrolytic cleaning of copper, the copper that is dissolved in the electrolyte from copper anodes precipitates as pure copper, by means of an electric current, on the surface of the cathode plate, from where it is typically removed mechanically in the form of a copper plate. In the recovery of copper, copper precipitates on the cathodes directly from the electrolytic solution. The cathodes are generally called permanent cathodes, because they can be re-used subsequently. It is well-known to use stainless steal as the material of the cathode plate. Generally, acid-proof steel 316 L is used, having a surface quality of 2B, whereby the Ra value describing the roughness of the surface is within a range of 0.3-0.6 μm. Typically, the surface mentioned above is obtained, when the steel plate working as the cathode is cold-rolled, annealed, etched in an acid bath, and subjected to dressing. In etching and dressing, the grain boundaries of the steel open, whereby microscopic grooves and canyons are formed on the surface of the steel plate, and the electrolytically coated metal is able to grow in and attach to these grooves and canyons. In use, however, the surface of the steel plate undergoes wear, contamination and changes, whereby the surface must be treated by brushing and grinding. In terms of process technology, manufacturing the surface of the cathode plate by etching and dressing is quite challenging, and an unacceptably short etching time can easily destroy the quality of the surface so as to become unsuitable for the electrolytic coating.
Publication FI 68430 B, for example, discloses the use of stainless steel as material for cathodes. Regarding the material, the publication describes in detail the AISI 316L steel, which has a surface treatment known as Standard 2B. The publication mentioned above states that the material in question is advantageous, as a sufficient fixing adhesion is provided between the steel plate and the copper so that the copper does not detach of its own accord before the actual releasing phase. The use of stainless steel as the material for cathodes is also disclosed in publication U.S. Pat. No. 6,485,621 B. Publication US 2006/0201586 A1 discloses a permanent cathode that is used in the electrolytic refining of metals, consisting of duplex steel containing a small content of nickel, or of steel grade “304”. The publication goes on to describe that the surface of the cathode plate is treated to improve its adhesion properties. The publication presents a surface treatment for improving the adhesion properties of the cathode surface, such as a mechanical treatment, e.g. grinding, to change the hardness of the surface, and etching. Furthermore, the publication suggests that the cathode surface be slotted to improve the above-mentioned properties.